In recent years, as the display market expands, a demand for requiring a clearer image is increased and, instead of a simple transparent material, an optical material having more advanced optical properties imparted thereto has been demanded.
In general, since the refractive index of a polymer is different in a molecular main chain direction and a direction perpendicular to the main chain direction, birefringence occurs. Depending on the purpose, it is required to precisely control the birefringence, and in the case of a protective film used for a polarizing plate of liquid crystals, a polymer material molded body in which the birefringence is smaller while the total light transmittance is the same is needed, and triacetylcellulose is used as a representative material.
In this situation, recently, as the size of a liquid crystal display is increased and the size of a polymer optical material molded article necessary for the liquid crystal display is increased, a material which exhibits little change in birefringence due to external force has been demanded in order to reduce a distribution of birefringence caused by deviation of the external force.
A material, in which a molded article which exhibits little change in birefringence due to external force is obtained, is a polymer optical material in which a molded article having a low photoelastic coefficient is obtained. Among these materials, a polycarbonate resin has been attracting attention as a material for an optical film used for the purpose of protecting a polarizing plate, since the polycarbonate resin excels in transparency and heat resistance and has excellent mechanical properties such as shock resistance. However, the polycarbonate resin has a disadvantage in that a phase difference (photoelastic coefficient) greatly varies with respect to a distortion when an external force is added, and thus the phase difference greatly varies even by a small distortion, thereby easily generating optical unevenness. As the polycarbonate resin which has a low photoelastic coefficient and can obtain a cured product suitable for use in the optical material, for example, disclosed is polycarbonate to which a fluorene skeleton is introduced (for example, refer to PTL 1). However, as described above, it is necessary to introduce a special skeleton, for example, a fluorene skeleton, for the polycarbonate resin disclosed in PTL 1, and thus there is a problem in that the cost is increased. Also, the Tg of the resin is 200° C. or higher, which is extremely high, compared to a general bisphenol-based polycarbonate resin whose Tg is about 140° C. Therefore, it is difficult to apply a melt extrusion method, which is a general film forming method of a polycarbonate resin film, and there is also a problem in handling properties.